Method and apparatus for mounting electronic parts

ABSTRACT

The present invention relates to a method and apparatus for mounting an electronic part (11) on a circuit board (28). When the part is mounted, a distance between the under surface of a part maintaining section (12) and the circuit board (38) is measured. Determination that the electronic part (11) is normally mounted on the circuit board (28) is made by comparing the measured distance with the thickness of the electronic part (11) measured in advance.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for mountingelectronic parts, particularly electronic parts formed into a chip-likeshape (which will be hereinafter referred to as chip part), on a circuitsubstrate such as printed circuit board.

DESCRIPTION OF THE PRIOR ART

In conventional electronic part mountings, vacuum absorption isgenerally employed as shown in FIG. 1 wherein a chip part 1 is attractedthrough a nozzle 2. FIG. 1 (a) to FIG. 1(d) are illustrations of theconditions of vacuum absorption of the chip part 1. FIG. 1(a) throughFIG. 1(d) respectively illustrate the condition that the chip part 1 hasnormally been absorbed by the nozzle 2, the condition that it haslengthwise been drawn thereby, the condition that it has obliquely beenabsorbed thereby, and the condition that it has not been attracted. Ofthese, FIG. 1(b) to FIG. 1(d) show error of the absorption of the chippart 1, and therefore detection should be made to determine theseconditions.

FIG. 2 is an illustration of a conventional vacuum system for thedetection of absorption error of a chip part. In the figure, thereference numerals 3 and 4 represent a filter, numeral 5 designating athrottle valve, numeral 6 depicting a magnet, numerals 7 and 8 being aproximity switch, and numeral representing a vacuum source. This systemis based on a method in which the absorption error is detected on viewof differential pressure. In response to the normal absorption of thechip part 1 to the nozzle 2, the pressure of the nozzle side of adifferential measuring device 10 is further decreased. As a result, themagnet 6 is moved in the direction of an arrow to thereby turn on theproximity switch 8. On the other hand, in the case of the occurrence ofabsorption error thereof as shown in FIGS. 1(b) to 1(d), the magnet 6causes no movement because air is introduced from an absorbing openingof the nozzle 2, resulting in turning on another proximity switch 7.

However, one of problems with such a method is that fine adjustment ofthe throttle valve 5 is required in conformity with the introductionamount varied in accordance with the absorbing condition of the chippart 1, taking a long time for system maintenance.

Various detection methods of the type other than the vacuum differentialpressure type of FIG. 2 are known. However, any of them relate to atechnique in which the detection is made on the basis of the degree ofvacuum, i.e., on the basis of air leak caused by the absorption error ofa chip part. Therefore, difficulty is encountered to set a detectionreference and further there arises an disadvantage in system maintenancethat the detection reference is varied due to the absorption of dust andthe like.

DISCLOSURE OF THE INVENTION

A primary object of the invention is therefore to provide a method formounting electronic parts which is capable of increasing reliability interms of the detection of a chip part and simplifying systemmaintenance.

Another object of the invention is to provide an apparatus forrealization of the method of the invention.

The above-mentioned object is accomplished by a technique wherein adistance between the under surface of a part maintaining device formaintaining an electronic part and a circuit substrate is measured whenthe electronic part is mounted on the circuit substrate and then acomparison is made between the measured distance and the thickness ofthe electronic part measured in advance. The determination that theelectronic part has normally mounted thereon is made in accordance withthe result of the comparison.

An electronic part mounting apparatus of the invention comprises asliding shaft having a part maintaining device at an end thereof, amotor for moving the sliding shaft upwardly and downwardly, a drive unitfor moving the sliding shaft within a horizontal plane, distancemeasuring means for measuring a distance between a circuit substrate andthe under surface of the part maintaining device when an electronic partis mounted on the circuit substrate, and determining means fordetermining a normal mounting of the electronic part by comparing themeasured distance with the thickness of the electronic part measured inadvance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1(a) to 1(d) are illustrations for describing the conditions thata chip part is absorbed by a nozzle;

FIG. 2 is a diagram showing a vacuum system based on a conventional chippart detection method;

FIG. 3 is a perspective view illustrating an electronic part mountingapparatus according to a first embodiment of the present invention;

FIG. 4 is a side view showing a main portion of a mounting section ofthe first embodiment;

FIGS. 5(a) to 5(c) are expanded views showing the conditions that a chippart is mounted;

FIG. 6 is a perspective view showing a second embodiment of the presentinvention;

FIG. 7 is a block diagram illustrating a linear motor control section ofthe embodiment; and

FIG. 8 is a flow chart showing a mounting operation of the embodiment.

THE MOST PREFERRED EMBODIMENT OF THE INVENTION

Hereinbelow, embodiments of the present invention will be described withreference to the drawings. A first embodiment of the invention will bedescribed with reference to FIGS. 3 to 5.

In FIGS. 3 and 4, the reference numeral 11 represents a chip part,numeral 12 designating a nozzle for absorbing the chip part 11, numeral13 depicting a hollow sliding shaft having the nozzle 12 at an endthereof which is guided by means of sliding bearings 14 to be slidableupwardly and downwardly, and numeral 15 representing a tube connectingbetween the sliding shaft 13 and a vacuum source.

A differential transformer 17 is fixedly secured to the sliding shaft 13by means of a bracket 16. A needle 18 of the differential transformer 17is secured through a compression spring 19 to a solenoid 20 which is inturn secured to the sliding shaft 13 by means of a bracket 21. Theabove-described absorbing section is allowed to run away upwardly byseveral millimeters.

Designated at numeral 23 is a linear motor comprising a body 23A and abobbin 23B having a wound wire. An application of a control signal tothe bobbin 23B causes upward and downward movement of the sliding shaft13 through a plate 24 mounted on the bobbin 23B. Numeral 25 represents adetector for detecting a position of the bobbin 23B in terms of height.The linear motor body 23A is fixedly secured at an end of a X-Y driveunit 26 and is selectively located in a X-Y position.

Numeral 27 designates a part supplying section for supplying a chip partand numeral 28 depicts a circuit board on which the chip part ismounted. Designated at 29 is a control unit including a microprocessorand a memory. The control unit 29 is arranged to position the linearmotor body 23A on X and Y axes, control upward and downward movement ofthe sliding shaft 13, and perform ON-OFF control of part absorption.

The operation of the mounting apparatus thus arranged will be describedhereinbelow.

First, The X-Y drive unit 26 is driven such that the nozzle 12 ispositioned above the part supplying section 27, and in response to thedrive of the linear motor 23 the nozzle 12 is fallen and lifted afterabsorption of the chip part 11. The X-Y drive unit 26 is further drivenso that the nozzle 12, having the chip part 11, is located at a mountingposition above the circuit substrate 28. Thereafter, the nozzle 12 isfallen in order to mount the chip part 11 on the circuit substrate 28.On mounting the chip part 11, current to the solenoid 20 is turned offwhereby the needle 18 is dropped under the action of the compressionspring 19. The differential transformer 17 is a well-known displacementsensor and is arranged such that its output voltage is varied inaccordance with the displacement amount of the needle 18. Therefore, itis possible to measure a distance between the under surface of thenozzle 12 and the circuit substrate 28, i.e., a height h at the time ofthe mounting of the chip part 11, on the basis of the variation betweenthe voltage before the falling of the needle 18 and the voltage afterthe falling thereof. After the termination of the measurement, theneedle 18 is lifted in response to application of current to thesolenoid 20, and the nozzle 12 is also lifted and transferred by meansof the X-Y drive unit 26 to the position of a part to be subsequentlymounted. The above-described operation is cyclically repeated, whereinthe mounting height h is subsequently measured while mounting.

FIGS. 5(a) to 5(c) are diagrams showing the conditions when the chippart 11 is mounted on the circuit board 28 by the nozzle 12. Of these,FIG. 5(a) illustrates the condition that the chip part 11 is normallymounted, FIG. 5(b) shows the condition that it is lengthwise mounted,and FIG. 5(c) shows the condition that it is not mounted because ofabsorption error. Assuming that the distance between the under surfaceof the nozzle 12 and the circuit board 23 is h1, and upper and lowerlimits taking into account the thickness of the chip part and thedispersion in terms of the thickness of the chip part are respectivelyh3 and h2, when the chip part is normally mounted, the relation betweenthem is h3≧h1≧h2. Therefore, the cases other than the relation result inthe mounting error of the chip part shown in FIGS. 5(b) and 5(c).

Secondly, the second embodiment of the present invention will bedescribed with reference to FIGS. 6 to 8.

The embodiment of FIG. 6 is similar to FIG. 3 embodiment with theexception that it does not include the compression spring 22 on thesliding shaft 13 not to have the structure capable of setting free thesliding shaft 13 upwardly and further it does not include the detectorfor measuring a distance between the under surface of the nozzle 12 andthe circuit board 28.

FIG. 7 is a block diagram illustrating a section of the control unit 29for controlling the linear motor 23 of FIG. 6. The reference numeral 40represents a linear motor corresponding to the linear motor body 23A andthe bobbin 23B of FIG. 6. Designated at numeral 41 is a position sensorfor detecting the position of the bobbin 23B which corresponds to adevice indicated by numeral 25 in FIG. 6. Numeral 42 depicts a poweramplifier for driving the linear motor 40, numeral 43 representing apreamplifier, numeral 44 designates a D/A converter for converting aposition command (digital signal) from the control unit into an analogsignal. Furthermore, numeral 45 represents an A/D converter forconverting a position feedback signal (analog signal) 46 for measurementof position into a digital signal, numeral 47 representing a D/Aconverter for converting a load command (digital signal) from thecontrol unit into an analog signal, and numeral 48 designating a modechanging switch for selecting either of the position control or loadcontrol of the linear motor 40.

FIG. 8 is a flow chart for the operation of mounting of the chip part.Hereinbelow, the operation will be described with reference to thefigure.

Initially, in a step 101, the nozzle 12 is moved to the part positionabove the part supplying section 27 by means of the X-Y drive unit 26.Subsequently, a step 102 is executed to fall the nozzle 12, followed bya step 103 in which the chip part 11 is absorbed thereby and a step 104in which the nozzle 12 is lifted with the absorbed chip part 11. Thenozzle 12, in a step 105, is moved to the mounting position above thecircuit board 28, followed by a step 106 in which the nozzle 12 isdropped up to the position that is separated by 0.5 to 1 mm from themounting position of the chip part 11. A subsequent step 107 is executedto switch the mode changing switch 48 to the load control mode, followedby a step 108 wherein the nozzle 12 is fallen in accordance with commandload. In a step 109, the position feedback signal 46 from the positionsensor 41 is A-D converted by the A/D converter 45 and measured is adropping amount of the nozzle 12 secured through the sliding shaft 13and the plate 24 to the bobbin 23B. Assuming that the dropping amount ofthe nozzle 12 is Hd and the distance between the lifted position of thenozzle 12 and the circuit board 28 is He, the height h1 of the mountedpart is expressed by h1=(He-Hd). He is known in advance, and therefore,if the dropping amount Hd of the bobbin 23B is measured in the step 109,it is possible to calculate the part height h1.

A step 110 is provided for comparing h1 with h2 and h3 indicated in FIG.5(a), and in accordance with the result of the comparison either of step111 or 114 is executed. If the determination is made as an abnormalmounting of the part, the step 114 is executed to lift the nozzle 12,followed by a step 115 in which an error process is performed. On theother hand, if normal, the step 111 is executed such that the nozzle 12is lifted. The step 111 is followed by a step 112 which is provided forchecking whether the mounting to the circuit board is perfectlyterminated. The termination causes the end of this operation, while notermination results in executing a step 113 wherein the nozzle 12 ismoved to the position of the part which will be subsequently mounted.Thereafter, the operational flow returns to the step 102.

This embodiment, as compared with the first embodiment, is arranged suchthat the height h1 on the mounting of the chip part 11 is calculated onthe basis o the output of the position sensor 25 necessary for theupward and downward drive control of the bobbin 23B by the linear motor23.

As described above, because the part height h1 is calculated bymeasuring the dropping amount of the nozzle 12 on part mounting on thebasis of the feedback signal 46 from the position sensor 41 for theupward and downward drive source of the chip part maintaining sectionwhich feedback signal 46 is A-D converted into a digital signal by meansof the A/D converter 45, it is possible to detect the height on the chippart mounting without providing a detector for detecting the part heightat a moving portion, resulting in reduction in cost.

In place of the differential transformer 17 used in the firstembodiment, an optical length measuring device can be used. It isappropriate to use any device which is capable of measuring the distancebetween the under surface of the nozzle 12 and the circuit board 28.

Furthermore, in the second embodiment, an analog output sensor has beenused, however, it is possible to use a digital output sensor of opticaltype or magnetic type. In this case, there is no need for using the A/Dconverter 45.

Industrial applicability

As described above, according to the present invention, the height of achip part is detected when the chip part is mounted and the partmounting is directly confirmed by comparing the detection value with therange of dispersion in terms of the thickness of the chip part.Therefore, it is possible to increase the reliability of the mountingdetection and to simplify the system maintenance, resulting in largepractical effect.

What is claimed is:
 1. A method for mounting an electronic part on acircuit board, comprising the steps of:measuring a distance between (a)the under surface of a part maintaining section for maintaining saidelectronic part and (b) said circuit board by a distance measuring meansattached to said part maintaining section; and determining a normalmounting of said electronic part by comparing the measured distance witha thickness of said electronic part measured in advance.
 2. A method asclaimed in claim 1 wherein said measuring step is accomplished when saidelectronic part is mounted on said circuit board.
 3. An apparatus formounting an electronic part on a circuit board, comprising:a slidingshaft having a part maintaining section at an end thereof; a motor formoving said sliding shaft upwardly and downwardly; a driver unit formoving said sliding shaft within a horizontal plane; distance measuringmeans for measuring a distance between said circuit board and the undersurface of said part maintaining section when said electronic part ismounted on said circuit board, said distance measuring means comprisinga differential transformer attached to said sliding shaft and having aneedle movable upwardly and downwardly and a solenoid for maintainingsaid needle at an upper portion; and determining means for determining anormal mounting of said electronic part by comparing the measureddistance with a thickness of said electronic part measured in advance.4. An apparatus as claimed in claim 3, wherein said part maintainingsection comprises a nozzle for absorbing said electronic part by vacuumabsorption at an end thereof.
 5. An apparatus for mounting an electronicpart on a circuit board, comprising:a sliding shaft having a partmaintaining section at an end thereof; a motor for moving said slidingshaft upwardly and downwardly, said motor comprising a position sensorfor outputting a position signal for controlling said motor; a driveunit for moving said sliding shaft within a horizontal plane; distancemeasuring means for measuring a distance between said circuit board andthe under surface of said part maintaining section by calculating adropping amount of said part maintaining section on the basis of saidposition signal of said position sensor when said electronic part ismounted on said circuit board; and determining means for determining anormal mounting of said part by comparing the measured distance with athickness of said electronic part measured in advance.
 6. An apparatusas claimed in claim 5, wherein said part maintaining section comprises anozzle for absorbing said electronic part by vacuum absorption at an endthereof.
 7. An apparatus as claimed in claim 6, wherein said positionsensor comprises a detector for detecting a position of a movable bobbinof said motor.
 8. An apparatus as claimed in claim 5, wherein saidposition sensor comprises a detector for detecting a position of amovable bobbin of said motor.
 9. An apparatus as claimed 5, wherein saidmotor comprises a linear electric motor.
 10. An apparatus as claimed inclaim 9, wherein said position sensor comprises a detector for detectinga position of a movable bobbin of said motor.
 11. An apparatus asclaimed in claim 9, wherein said part maintaining section comprises anozzled for absorbing said electronic part by vacuum absorption at anend thereof.
 12. An apparatus as claimed in claim 11, wherein saidposition sensor comprises a detector for detecting a position of amovable bobbin of said motor.